Due to their good ratio of rigidity or strength to density, composite materials, and in particular sandwich constructions, are widely applied in many areas of aircraft engineering. Generally speaking, sandwich constructions are made from a top face sheet or face layer and a bottom face sheet or face layer, between which layers or sheets, for the purpose of increasing rigidity, there is a honeycomb-like core structure formed by vertically extending cells with hexagonal cross sections.
The specific mechanical potential of cellular materials, when compared to such honeycomb structures, is lower due to their structure. Nevertheless, above all in the manufacture of components and in the region of expanded component characteristics, cellular materials are of interest due to their multifunctionality for application in sandwich constructions for structural aeronautical applications. For this reason various experiments are being carried out in an attempt to improve the mechanical characteristics of the cellular materials without incurring an excessive increase in density. By means of sewing techniques, the incorporation of pins, or by means of similar methods, the core is locally reinforced without incurring a considerable increase in the weight of the structure. Moreover, in various methods there is an option, by means of local variation in the pin density and in the pin angle, of tailoring the mechanical characteristics of the core structure to a particular case of application as well as tailoring them locally. Apart from the mechanical characteristics that are of interest from the point of view of statics, these core structures in addition comprise characteristics that are very interesting as far as the impact or the degree of impact damage is concerned. For example, in reinforced cellular materials a crack-stopping effect can be detected.
Among other things in the region of thermal and acoustic insulation, as well as in their manufacture, these sandwich constructions comprising a high-resistance cellular material core are associated with advantages when compared to honeycomb structures, but they are associated with disadvantages in that they comprise only comparatively poor mechanical characteristics. In order to compensate for these mechanical disadvantages, sewing techniques are used by means of which it becomes possible to incorporate fibres, fibre bundles or threads in high-resistance cellular material components. After a resin infiltration process, the regions interspersed with fibres then contribute to the mechanical reinforcement of the cellular material.
The sewing methods that are commonly used to reinforce cellular materials consist of penetrating a cellular material by means of a needle, and in this process at the same time of pulling the thread or the fibre bundles or fibres through the high-resistance cellular material. Two different methods are used to fix the thread.
Firstly, by means of the sewing method known as tufting, a thread can be pulled through the high-resistance cellular material layer, and can be affixed to a substrate, for example silicon rubber, situated on the opposite face. After completion of the seam the substrate can be removed.
The second sewing method belongs to the category of double-face sewing methods, wherein an upper thread from a face layer of the sandwich construction is stitched through the layer construction with a needle. Subsequently the upper thread is affixed, by means of a bottom thread, to the opposite face of the layer construction.
Due to the fact that the needle and the thread enter the cellular material at the same time, a hole size is generated in the cellular material, which hole size is larger than the diameter of the incorporated fibre quantity. For example, if the high-resistance cellular material is further processed, for example during infiltration, the remaining void of the holes, which void is not filled by fibre bundles, is filled by the resin.
The known sewing methods have one aspect in common, namely that first a needle penetrates the cellular material and in this process at the same time incorporates the thread in the cellular material. In this process during insertion in the cellular material the thread extends parallel to, and essentially over the entire length of, the needle. The hole size of the insertion hole is thus determined by the needle diameter and the thickness of the thread.
All these known methods are associated with a disadvantage in that after withdrawal of the needle from the cellular material the remaining hole is too large in relation to the thickness of the incorporated thread. This leads to a situation in which after infiltration with a resin the hole region that is not taken up by fibres is filled with resin, and consequently the improvement in the mechanical characteristics is not implemented by the fibres, as desired, but instead, depending on the method, is essentially implemented by the incorporated resin. However, the improvement of the specific, i.e. weight-related, mechanical characteristics is insufficient, when compared to those of honeycomb structures, for the light-weight construction potential necessary in aircraft engineering, so that the use of cellular materials reinforced in this way can only seldom be considered.
In order to illustrate the importance and the advantages of the present invention, the following should be added in the context of the technical field of manufacturing reinforced materials:
Investigations of the effects of titanium pin reinforcements on the failure pattern of the sandwich construction have shown that in the case of reinforced cellular materials the area of damage clearly remains limited to the region within an inner row. It is thus clear that the damage is locally confined. During further investigations the effects of the space between reinforced regions can be determined. If, in a relative dense reinforcement, failure of the face sheets is due to complex interaction of local and global flexing and shear failure of the face sheets, when the rigidity is reduced, due to the lower reinforcement density the face sheet failure is dominated by bending. The damage pattern shows localised damage and micro cracks in the impacted face layer, but no damage on the reverse face. In the region of the impact the pins that have penetrated the face layers have been pulled out. Furthermore, fibre rupture occurs, as does local separation of the core from the face layer in the region of impact. These practical results agree well with theoretical simulations. Also in this context CAI investigations can be made; they show that in the case of a non-reinforced cellular material the main failure mechanism consists of microbuckling of the face sheets. However, in the case of reinforced cellular materials, separation/release of the pins is the main failure mechanism. Apart from the NDT behaviour of reinforced cellular materials it is also possible to investigate the dependencies on the reinforcement angle. One result demonstrates that the limiting value for the introduction of damage as a result of pulling the pins from the face layer depends strongly on the pin angle. In the case of a 10° reinforcement the limiting value at which damage that is worth mentioning occurs is more than twice the value in the case of a 20° reinforcement. Investigation (both experimental and by means of FEM-analysis) of the energy absorption capacity of reinforced cellular materials subjected to pressure loads shows that by increasing the thickness the energy absorption capacity can be greatly increased. It is important to ensure that the space between reinforcement elements is less than half the wavelength of the folds that are created in a non-reinforced sandwich construction of the same design.
Reinforcement by Means of Stiffened Pins:
In industrial development projects a new core material has been developed that corresponds to the characteristics of the 48 kg/m3 honeycomb while saving 10% in weight. This new core comprises a light cellular carrier material that is reinforced by thin pins in order to improve its structural characteristics. The reinforcing semi-finished products are thin bar-shaped elements of any desired cross section, provided they comprise adequate inherent stiffness because otherwise they cannot be processed. The diameter of the pins used is between 0.279 and 0.711 mm Taking into account the respective materials characteristics the pins can be from any of the three materials categories, for example fibre-reinforced plastic, titanium alloys, glass, Nicalon or quartz. In the method developed, the pins are shot, with the support of ultrasound, into the cellular material, and in a second step they are transformed at the surface. The resulting product is marketed by the trademark of K-COR™. As an alternative to the above the pins can also enter the face layer. This product is commercially available by the name of X-COR™. This method provides a very considerable advantage in that the semi-finished reinforcement products can be manufactured in a separate process step as an endless product. Especially in the case of semi-finished bonded fibre fabrics, whose characteristics depend greatly on the fibre volume content and the fibre orientation, this is very positive. Designers thus have the option, by varying the local pin density, pin length, pin diameter and pin angle, to design a core that is optimal for each application. Possible angles range from vertical pins for component regions that are particularly strongly subjected to pressure, to angles between 20° and 30° for shear reinforcement.
Reinforcement by Means of Dry Semi-Finished Products:
Dry reinforcement of cellular materials is possible using various methods: sewing methods, winding-/braiding methods and stapling methods. The resulting products differ greatly both in the quality and in the flexibility of their reinforcement. Finishing of the dry-reinforced cellular material cores takes place in a subsequent infiltration process.
Sewing Methods:
There are two sewing methods that differ in principle: namely the single-face sewing methods with only an upper thread (e.g. tufting, blind stitching), and the double-face sewing methods comprising an upper thread and a bottom thread.
First we will discuss the double-face sewing methods. Generally speaking, various stitch types are known from textile processing. Examples include the lock stitch and the chain stitch.
Of these types of stitches the double lock stitch has been shown to be most suitable for reinforcing a cellular material. To form the double lock stitch an upper thread and a bottom thread are used in the textile industry, also referred to as needle thread and gripper thread. The needle thread is kept in the needle by means of the eye of the needle, which is situated in the tip of the needle, and is stitched through the component. During the reverse movement of the needle, the needle thread forms a loop that is gripped by the gripper tip. As a result of the rotational movement of the gripper the loop is enlarged and pulled around the gripper. In this process the needle thread loop is placed around the looper thread so that the latter is affixed. The position of the looping point is set by way of the thread tension. In the textile industry it is common, by means of identical upper thread and bottom thread tension, to position the knotting point in the middle of the goods to be sewn. In this way, among other things, an increase in the stretching ability of the seam is achieved. With the use of the double lock stitch in bonded fibre technology, this mid-point arrangement of the knot results in a host of undesirable side effects. Pulling the thread through the substrate increases the already arising undulation of the fibres in the placed scrim. However, since the interaction of bonded fibre fabric depends very strongly on the defined alignment of the fibres in the laminate, any interference, although unavoidable, is to be kept to an absolute minimum.
A further side effect refers to the sewing-together of bonded fibre fabric textiles; apart from fixing the individual layers such sewing-together also makes it possible to improve the interlaminar shear strength, i.e. reinforcement in the third dimension. The looping point is a weak point in this reinforcement and should therefore if at all possible be situated outside the effective region. For the reasons mentioned above, in bonded fibre fabric technology the looping point is placed to the bottom face of the laminate by increasing the bottom thread tension. As far as the yarns to be processed are concerned, it must be taken into account that during stitch formation the sewing thread is subjected to considerable friction loads and transverse loads. Consequently only yarns providing adequate flexural strength (for example Kevlar) can be processed without any problems. The use of rovings is possible only with extreme difficulties, or sometimes it is not possible at all. The described principle of creating a double lock stitch in a semi-finished textile product cannot be transferred without modifications to the reinforcement process of semi-finished cellular products, namely due to the high substrate height relative to textiles. For this purpose corresponding equipment was developed in corresponding research projects.
In an English sewing device, for example, the individual sewing needles are replaced by a needle bar by means of which several stitches can be made at the same time. The gripping system on the bottom face of the substrate is substituted by a principle from the field of projectile weaving looms. After stitching, the loops of the upper threads are opened up on the bottom face, and the bottom thread is shot through all the loops. Investigations have, among other things, been carried out on components that were reinforced by means of the double lock stitch. In the case of reinforced cellular materials the surface of a separated face sheet reduces considerably after the effect of an impact, wherein, depending on the stitch density, the damage visible from the outside is only slightly less than the inner damage. The amount of absorbed energy first increases until it decreases when the face sheet is perforated. Further investigations relating to the behaviour of cellular materials that were reinforced with the use of sewing techniques have shown that with this type of reinforcement there is an increase in the damage tolerance, as there is an increase in the nominal mechanical characteristics, but that the increase in weight is not insubstantial. Single-face sewing methods are associated with a very considerable advantage when compared to the already described double-face sewing methods in that the component needs to be accessible only from one face. Blind stitching and tufting are, for example, possible sewing methods.
Due to stitch formation, blind stitching is unsuitable as a reinforcement method. Tufting as a sewing method is related to double lockstitching, except that the bottom thread is replaced by a substrate, for example silicon, in which the formed loop is fixed when the needle is withdrawn.